1. Field of Use
This application relates to the measurement of air pollution and in particular to the rapid measurement of the quantity and size distribution of aerosol particles. As vehicle engines become more complex and varied, it becomes necessary to have better systems to determine our motor vehicle emissions inventories. To develop accurate ultrafine particle models, the common practice of using engine dynamometers and in-lab testing will need to be replaced with in-situ monitoring of vehicles on the road. However, measurement of engine exhaust particle size is currently done using instruments that are too bulky, expensive, and power inefficient to easily adapt to on-board, in-situ particle measurement.
2. Description of Prior Art (Background)
There are several limitations with current systems for measuring engine exhaust particles, in particular ultrafine particles, or particle diameters less than 100 nanometers. Measuring ultrafine particulate is typically done in a laboratory setting. Particulate monitoring instruments are bulky and not designed for in-situ (i.e., on board and real-time) particulate monitoring. Those particulate sizing instruments are generally connected to engine dynamometers which are operated at loads to roughly simulate on-road conditions and are not suitable for in-situ fleet-wide monitoring of engine exhaust particles.
In one optical system, light is directed through aerosol particle-laden smoke and the attenuation of the light is measured on a detector to indicate total particle concentration. This method does not measure particle size distribution, however. Another optical method uses light scattering to measure particle size by causing the particles to pass one at a time through a chamber so that scattered light amplitude depends on the particle size. The amplitude is measured by a photomultiplier which produces an electrical signal dependent upon particle size. To isolate single particles for detection, gas sampling must be done at low velocity, and the system is usually provided with very narrow pipes which are subject to contamination, require frequent cleaning, and tend to collect the larger particles before their entry into the sensing chamber. Further, such method of measuring the size of a single particle is quite slow, requiring perhaps as much as an hour for a typical measurement.
Electrical methods have the advantage that they can be operated nearly continuously with the results available to the operator after a very short interval of time. In one electrical method described in U.S. Pat. No. 3,114,877 to Dunham, a charging device operates to charge separate groups of aerosol particles passing the device. The particles then flow in a random manner through a field-free region, pass an ion trap and flow to a detector. At the detector, the particles lose their charge and produce a current. Although the detector current in the Dunham apparatus is said to be an index of the number of particles, it is clear that the amplitude of the current is a function of the total charge on all of the particles sensed by the detector at a given moment. Thus, the amplitude of the current is a function of the total surface area of the particles. Because the particles flow in a random manner to the detector, particles having different surface areas (and thus different sizes) lose their charge at the same moment of time to produce the current. Therefore, the output current in the Dunham apparatus is not indicative of the number of particles except when they are of uniform size.
Another method which indicates aerosol particle size distribution is based on the mobility of charged particles in an electric field extending radially across a tube in which the particles flow. Mobility is a measure of the velocity of a charged particle in an electric field, and generally speaking, the higher the charge on the particle the higher the mobility. For a given method of charging a particle, the amount of charge on the particle is a function of the size of the particle. Therefore, mobility is a function of particle size and methods based on particle mobility utilize the difference in mobility to measure particle size distribution. In one such device described in U.S. Pat. No. 3,413,545 to Whitby, clean air is caused to move downwardly in an annular flow path surrounding an elongated electrode extending axially in a cylindrical housing. Charged aerosol particles are introduced around the outer periphery of the flow path of clean air and an electric potential is applied across the elongated electrode and the cylindrical housing. For any given potential, particles having mobility below a certain value will not move far enough radially to contact and lose their charge to the elongated electrode before passing its downstream end. An electrometer detects these charged particles which generate a current, the amplitude of which is a function of the total charge on the detected particles. By varying the potential applied to the elongated electrode, more or fewer charged particles will reach the detector and induce the current. By relating the current produced when various potentials are applied to the elongated electrode, a measure of particle size distribution can be obtained. However, a number of factors limit the usefulness of this device for monitoring effluents in stacks of industrial installations, for example. Due to the method of charging, known as diffusion charging, only particles less than about 2 microns diameter can be measured whereas in a typical stack, particles up to 100 microns or more will be present. Further, the diffusion charging method is also inconvenient because it requires a source of compressed air and various thin pipes which are subject to clogging.
Accordingly, there is a need for a method and apparatus for a compact, low-cost, low power system capable of discriminating and measuring in-situ particle size distribution based on particle mobility in an electric field utilizing a small volume differential mobility analyzer and disposable electrodes.